Barnacles

Imagine spending most of your life standing on your head and eating with your feet! Sound like a difficult way to get through the day? Well, that's exactly how barnacles spend most of their lives.

If you walk along the sea shore, you can find barnacles on almost any solid surface that gets covered by water. On rocks, dock pilings, boats, even mussels, you can find clusters of these hard, white, cone-like houses. That's where barnacles live, peeking out only when water covers them so they can filter food into their homes. This "barnacle zone" is the highest of the intertidal zones.

Although they may look like mollusks with their shell-like covering, barnacles are actually crustaceans, related to lobsters, crabs and shrimp. They look like tiny shrimp in their larval stage, where they swim as members of zooplankton in the ocean. When they are ready to settle down, they search for a suitable site, pulling themselves along by the adhesive tips of their antennae. Biologist have observed barnacles in the laboratory taking as long as an hour to pick a location. In nature, barnacles may take days to find a suitable spot, investigating one area, then allowing the currents to carry them to another.

After selecting a spot, the barnacle secures itself head-first to the surface with a brown glue. (This glue is so strong, the barnacle's cone base is left behind long after the creature has died. Dentists are now studying this glue for its adhesive properties.) Now the larva is ready to grow into an adult and build its tough housing.

From "The Intertidal Zone." Courtesy of Bullfrog Films and the National Film Board of Canada.

The barnacle secretes the calcium-hard plates which totally encase them. These white cones have six nearly fitted plates that form a circle around the crustacean. Four more plates form a "door" which the barnacle can open or close, depending on the tide. When the tide goes out, the barnacle closes shop to conserve moisture. As the tide comes in, a muscle opens up these four plates, and the feathery legs of the barnacle sift the water for food. All six pairs of these feather-like feeding appendages, called cirri, are jointed and set with sensory hairs which brush through the water collecting plankton for the barnacle to eat. The legs also have gills for gas exchange.

(From "The Intertidal Zone." Courtesy of Bullfrog Films and the National Film Board of Canada)

The more the barnacle eats, the more it grows. Not only does the growing barnacle have to deal with its surrounding house, it is also constrained by its own shell (made from chitin, the same material as lobster shells and insect exoskeletons). And that can create a tight living arrangement for the creature. Like lobsters, barnacles shed their shell when it gets too small for them. But since they never leave their plated homes, they must enlarge their current one. No one is quite sure how the barnacle accomplishes this home renovation, but there is probably a chemical secretion that dissolves the inner layers while new material is added to the outside.

Most barnacles are hermaphrodites, meaning they have both male and female sex organs. But to create baby barnacles, they must be fertilized by a neighbor. A retractable tube containing sperm can reach outside the shell as far as several inches to fertilize a nearby barnacle.

From "The Intertidal Zone." Courtesy of Bullfrog Films and the National Film Board of Canada.

(From "The Intertidal Zone." Courtesy of Bullfrog Films and the National Film Board of Canada.)

Newborn barnacles emerge from their parent's shells as one-eyed larvae. They feed on plankton voraciously, growing and molting into non-feeding, shrimp-like larvae. These larvae settle to the bottom, and begin feeling around for a new home, beginning the cycle all over again.

Barnacles feed on plankton they sweep from the water with their fan-like feet. Their main predators are whelks--snails that envelop the barnacle's cone and force the valves open. The blue mussel is a competitor for space, possibly outgrowing and smothering barnacles. Excessive growth of the algae rockweed can also overpower a colony of barnacles.